I found a couple sites for you to resource:
Zazzle has an outrageous collection of postcards. Although, I can't say if they're truly vintage. But if the look is what you're going for, this could be right up your alley.
Finally, if you wanted to have something off the charts amazing- have your own postcards commissioned. I love illustrator Jenny Medved for projects like these. We recently had her design the cover to one of our guest guides (for the welcome bags) and she is absolutely brilliant to work with.